The jersey, which is also signed by the Dodgers pitcher with the inscription "1963 W.S. Champs," is from that magical 1963 season where he won the Cy Young Award, Most Valuable Player and World Series MVP.
Darren Rovell of ESPN provided an image of the jersey before it was sold:
Koufax won three Cy Young awards in his career, but his first came in 1963, along with his only MVP award.
While it's unknown which game the jersey came from, authenticators have confirmed its legitimacy as a game-worn uniform.